feat(isMobilePhone): add en-GM locale for The Gambia

[ginxx009]

Adds en-GM mobile phone validation for The Gambia.

Gambia uses country code +220. National mobile numbers are currently 7 digits, and PURA is migrating Africell, QCell and Comium to 9 digits from 4 September 2026 (dual-dial period through 30 November 2026). Gamcel stays on 7 digits for now.

7-digit mobile prefixes

• Africell: 2xx, 7xx, 40x, 41x
• QCell: 3xx, 50–53, 58–59
• Comium: 6xx
• Gamcel: 9xx

9-digit PURA prefixes (existing 7-digit number plus operator prefix)

• Africell: 87 + 7 digits
• QCell: 83 + 7 digits
• Comium: 86 + 7 digits

Regex:

'en-GM': /^(\+?220)?(?:(?:2\d|3\d|40|41|5[0-38-9]|6\d|7\d|9\d)\d{5}|(?:83|86|87)\d{7})$/,

Accepts optional +220 / 220, current 7-digit mobiles, and the new 9-digit forms. Rejects landline blocks such as 42x/43x and other 8x prefixes that are not part of the PURA mobile migration.
Checked master and open PRs; no existing en-GM locale.
Refs #1761

https://en.wikipedia.org/wiki/Telephone_numbers_in_the_Gambia
https://pura.gm/public-notice-migration-of-the-national-mobile-numbering-plan-from-7-digit-to-9-digit-format/

## Checklist

- [X] PR contains only changes related; no stray files, etc.
- [X] README updated (where applicable)
- [X] Tests written (where applicable)
- [X] References provided in PR (where applicable)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (a79ff98) to head (88cd6a6).

Additional details and impacted files

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  Coverage   100.00%   100.00%           
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  Files          114       114           
  Lines         2599      2599           
  Branches       658       658           
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  Hits          2599      2599           

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